Human temporal bones obtained at autopsy from patients with histories of sensorineural hearing loss due to ototoxic drugs, noise exposure, aging, vascular disorders, and hereditary deafness are studied by microdissection and phase-contrast examination of surface preparations of inner ear tissues. Pathological findings in the form of cytocochleograms are correlated with audiometric data. Similar comparisons are carried out in macaques and other genera of monkeys trained for behavioral audiometry and treated with aminoglycosides or exposed to various noise spectra. Similarly, ototoxic drugs, noise exposure, and combinations of these cochlear insults are used in guinea pigs trained for behavioral audiometry and/or equipped with indwelling round window electrodes of chronic recording and analysis of the compound action potentials of the cochlear nerve. Pathological changes in the inner ears of experimental animals are examined by light and electron microscopy, including the cochlear and vestibular neurons. In selected animals, cochlear and vestibular nuclei are examined for possible degenerative changes. Aged monkeys and dogs are used as models for presbyacusis, and Dalmation dogs as models for hereditary cochleo-saccular degeneration. A study of the effects of cochlear prostheses on inner ear structures is being made in the temporal bones of patients and monkeys that have received such implants, as they become available at autopsy.